Intérêt des kystes de dinoflagellés calcaires et du biomicrofaciès pélagique dans la datation des terrains du sommet du Jurassique et de la base du Berriasien dans le domaine téthysien occidental

Abstract

The study of several hundreds of thin sections from dozen outcrops in three different paleogeographic basins: the External Rif Range (Morocco), the Vocontian Basin (Southeastern France) and the northern Tunisia, has confirmed the importance of pelagic microfaunal associations to date the Upper Jurassic deposits. Among those palaeontological groups, crinoid saccocomids, which appear in the Middle Oxfordian, are thriving noticeably in the three basins at the base of the lower Tithonian substage (Hybonotum ammonite zone). Calcareous dinoflagellate cysts (two new species) were also used to characterize 10 biozones, 7 at least have a general value in the Tethyan realm. Protoglobigerines, abundant in the Upper Oxfordian and at the base of the Kimmeridgian (Platynota zone), are rare or absent in younger strata. In the external Rif and in northern Tunisia, microfilaments show two levels of abundance, respectively in the Divisum and the Beckeri zones. They disappear at the top of the latter zone. Chitinoidellids were used to characterize the Lower Tithonian (Ponti zone)-Upper Tithonian (zone Microcanthum) boundary. Calpionellids, appearing at the top of the Microcanthum zone, are still abundant within the Durangites zone. Globochaete, often present, are really abundant only from the base of the Lower Tithonian and higher up throughout the Tithonian. Radiolarians show random stratigraphic distributions. Combining all these various fossil groups and their abundance, we define 13 biozones of concurrent range, which are well-correlated with ammonite zones. It corresponds respectively to the Upper Oxfordian-basal Kimmeridgian (Platynota zone), the Hypselocyclum zone, the Divisum zone, the Acanthicum-Eudoxus zones, the Beckeri zone, the Hybonotum zone, the Darwini-Semiforme zones, the Fallauxi zone, the Ponti zone, the Microcanthum pro parte zone, the upper part of the Microcanthum zone-lower part of the Durangites zone, the upper part of the Durangites zone and the lower part of the Euxinus zone. The present work aims to contribute towards to a better and more practical biostratigraphical zonation of the Upper Jurassic-lowermost Berriasian deposits in the western tethyan basins.